diff --git a/worker/lyra_worker/adapters/fakes.py b/worker/lyra_worker/adapters/fakes.py
index 83ab6fe..a97072b 100644
--- a/worker/lyra_worker/adapters/fakes.py
+++ b/worker/lyra_worker/adapters/fakes.py
@@ -2,6 +2,7 @@ from typing import Callable
 
 from lyra_worker.browser import ArtistHit, ReleaseGroupInfo
 from lyra_worker.probe import ProbeResult
+from lyra_worker.similarity.base import SimilarArtist
 from lyra_worker.types import Candidate, DownloadResult, MBTarget, Quality
 
 _QUALITIES = {
@@ -90,3 +91,19 @@ class FakeAudioProbe:
         self._r = ProbeResult(artist=artist, album=album, fmt=fmt, quality_class=quality_class)
     def probe(self, path: str) -> ProbeResult:
         return self._r
+
+
+class FakeSimilaritySource:
+    """In-memory SimilaritySource for tests. `similar` maps seed mbid -> [SimilarArtist]."""
+
+    name = "fake"
+
+    def __init__(self, similar=None, healthy=True):
+        self._similar = dict(similar or {})
+        self._healthy = healthy
+
+    def health(self) -> bool:
+        return self._healthy
+
+    def similar_artists(self, mbid: str) -> list[SimilarArtist]:
+        return list(self._similar.get(mbid, []))
diff --git a/worker/lyra_worker/similarity/__init__.py b/worker/lyra_worker/similarity/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/worker/lyra_worker/similarity/base.py b/worker/lyra_worker/similarity/base.py
new file mode 100644
index 0000000..fef5675
--- /dev/null
+++ b/worker/lyra_worker/similarity/base.py
@@ -0,0 +1,22 @@
+from dataclasses import dataclass
+from typing import Protocol, runtime_checkable
+
+
+@dataclass(frozen=True)
+class SimilarArtist:
+    mbid: str
+    name: str
+    score: float
+
+
+@runtime_checkable
+class SimilaritySource(Protocol):
+    name: str
+
+    def health(self) -> bool:
+        """Is this source reachable right now?"""
+        ...
+
+    def similar_artists(self, mbid: str) -> list[SimilarArtist]:
+        """Artists similar to the given artist MBID (MBID-native)."""
+        ...
diff --git a/worker/tests/test_similarity_base.py b/worker/tests/test_similarity_base.py
new file mode 100644
index 0000000..805d626
--- /dev/null
+++ b/worker/tests/test_similarity_base.py
@@ -0,0 +1,14 @@
+from lyra_worker.adapters.fakes import FakeSimilaritySource
+from lyra_worker.similarity.base import SimilarArtist, SimilaritySource
+
+
+def test_fake_conforms_to_protocol_and_returns_similar():
+    src = FakeSimilaritySource(similar={"seed": [SimilarArtist("c1", "Cand", 0.9)]})
+    assert isinstance(src, SimilaritySource)
+    assert src.health() is True
+    assert src.similar_artists("seed") == [SimilarArtist("c1", "Cand", 0.9)]
+    assert src.similar_artists("unknown") == []
+
+
+def test_fake_can_report_unhealthy():
+    assert FakeSimilaritySource(healthy=False).health() is False